The present invention generally relates to systems powered by an induction machine and, more specifically, to a method for protecting induction machines and its drive system from overcurrent.
Traditional aircraft power distribution systems employ a constant-frequency source of AC power in the distribution network. Electrical motors may be directly coupled to the AC bus with the result that there can be a large inrush of current at the start up. As electric control systems for aircraft become reality, for example, it is contemplated that the conventional constant-frequency AC bus used in the aircraft will likely be replaced by a variable-frequency electrical system. However, for a variable-frequency system it would not be feasible to directly couple an electrical machine to the variable frequency AC bus. A preferred configuration is to connect electrical equipment to the AC bus via a rectification and inverter system. As a consequence, for aircraft that utilize constant-voltage, variable-frequency distribution systems, it is desirable to minimize the weight and the size (i.e., the KVA rating) of the inverter so as to minimize the weight and cost of the distribution system.
When a linear ramp of the magnitude of the voltage with respect to frequency, a parameter commonly expressed in constant Volts per Hertz (Volts/Hz), is applied to an induction motor, it has been observed that the magnitude of the phase current does not remain constant. One of the chief reasons for this phenomenon is the observation that the slip frequency does not remain constant. FIG. 1 is a control block diagram of a conventional open-loop method of powering an induction machine 10 via a pulse width modulation operation 13. An input 11 is applied as a constant input voltage to frequency. This voltage rate of change is kept constant until the frequency reaches a desired steady state value. For applications where a rapid acceleration is required, the magnitude of the current drawn by the motor is not controlled and, hence can vary. In some instances, the variation in current can be very large. Accordingly, the induction machine system may be configured to accommodate large current levels such as by, for example, increasing the ratings of components such as Insulated Gate Bipolar Transistors (IGBT) used in the system inverter design.
For applications where rapid acceleration is required, the current drawn from an inverter (not shown) may become quite large compared to its nominal operating points. A nominal operating point may be defined, for example, as a peak power point at which the induction machine 10 is operating at its rated maximum speed and maximum torque. To accommodate this current demand, the rating of the inverter is typically designed for a worst case operating condition in which the current demand is a multiple of the steady state rating. The magnitude of the phase current could be dependent upon many factors including the ambient air temperature, altitude (i.e., air pressure), and load torque. For example, when a ‘fan’ load is applied to the induction machine 10, that is, a load in which torque increases as a square of the speed of the induction machine 10, the torque of the induction machine 10 will drop as a function of decreasing air pressure. Hence it is quite desirable to design a system where the inverter current rating is limited to a certain value during the acceleration to prevent the current reaching very high values.
U.S. Pat. No. 5,247,237 discloses an open-loop control device for protecting an induction motor from overcurrent. In a basic mode of control, the control device selects a rate of change of frequency that serves to limit the acceleration of the induction motor and acts to maintain the motor current below a limit value. If the current exceeds this limit value, a correction frequency is calculated and subtracted from an open loop frequency detected in a primary frequency command generator. Based on the corrected frequency, a voltage calculation is made and an appropriate command is provided to a power conversion circuit. However, an operator is required to select an arbitrary frequency ramp rate for the device of the '237 patent.
Such conventional control methods suffer from the shortcoming that the primary control variable is not designed to accelerate a machine at the maximum rate compatible with the load that is applied to the induction motor and with the capabilities of the inverter. What is needed is a method that can be utilized under all load conditions and applied voltage conditions using the full current capability of the inverter, regardless of the load that is applied to the motor.
As can be seen, there is a need for an improved apparatus and method for limiting the current of an induction motor placed under an acceleration demand.